1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device using a power supply voltage and other operation voltages.
2. Description of Related Art
Electronic apparatuses such as a personal digital assistant (PDA), a notebook computer or a mobile phone operate with a power supplied from a battery. Users desire to use these electronic apparatuses for a longer time without recharge, which can be achieved by lowering operation current or operation voltage. In addition, state-of-the-art electronic apparatuses require higher operation speed. These state-of-the-art electronic apparatuses employ semiconductor integrated current devices that include transistors such as metal-oxide-semiconductor (MOS) transistors. In order to achieve higher speed, it is desirable for a gate insulation layer of a MOS transistor to be formed so as to be of a relatively thin layer.
Additionally, an electronic apparatus operating with a lower operation voltage includes a number of semiconductor integrated circuit devices. The semiconductor integrated circuit device typically receives an external power supply voltage, may have an internal power supply voltage, and may uses a voltage higher than the external or internal power supply voltage. A relatively thick gate insulation layer is used in a MOS transistor operating with a high voltage, so as to enhance a withstanding voltage relative to the high voltage. On the other hand, a relatively thin insulation layer is used in a MOS transistor operating at a voltage other than (such as less than) the high voltage. Such an insulation system is referred to as a “dual insulation system”.
In the case where a relatively thick gate insulation layer is employed, it is possible to prevent the gate insulation layer from being broken due to a gate-drain voltage difference of a MOS transistor. The thicker the gate insulation layer is, the higher a threshold voltage of the MOS transistor. When the threshold voltage of the MOS transistor rises, turn-on speed of the MOS transistor may be lowered. This may cause a reduction in system operation speed.